1. Field of the Invention
The present invention relates generally to a method for detecting corrosion fatigue cracks in membrane boiler tubes, and more particularly to a method for detecting corrosion fatigue cracks in membrane boiler tubes using EMATS.
2. Description of the Related Art
Corrosion fatigue is a form of failure mechanism found in boiler tubes in fossil fired utility boilers. The mechanism for corrosion fatigue results from the interaction of mechanical stress and corrosion. Cyclic stressing is more damaging than constant stress and as a result there are more failures of this type. The actual damage consists of crack initiation and growth from the water side surface (I.D.) of the boiler tube. Multiple cracking usually occurs with one crack dominating causing a through wall failure. Cracks are typically wide, oxide filled, and appear with an irregular bulge profile. The damage resulting from these cracks is particularly serious in universal pressure (UP) units which are also known as once-through boilers.
The crack growth usually is in a direction perpendicular to the maximum tensile stress and depending on the particular stress situation, stress assisted cracking may be longitudinal, circumferential, or occasionally inclined at some angle. Typically in the UP waterwall panels the cracking is longitudinal. The tubes in the waterwall panels of fossil units have cracks that are ID initiated, radially oriented, with the failures typically occurring at the membraned welds or 90.degree. from the membrane welds on either the furnace side or the casing side.
In the past, if this problem was suspected in the utility boiler, the primary method of detection was radiography. Unfortunately, there are health hazards associated with radiography and due to the radiation large areas needed to be evacuated during inspection. Consequently, there have been various attempts to locate this type of damage by ultrasonic techniques. One conventional ultrasonic technique proved to be slow and thus could only serve as a survey technique for the high risk areas in the boiler. Even still, there were serious problems involved with poor signal-to-noise ratios evident on tubes with diameters smaller than 150 mills. A major problem associated with the ultrasonic test technique was that cracks can occur with equal probability on both the furnace and casing side of the membrane tube panel. The ultrasonic technique involved propagating ultrasound past the membrane which has a complex geometry. The other practical problem with the ultrasonic method is the size of the boiler tubes. Due to the small size of the tubes, there is very little room for an ultrasonic wedge.
Thus, there is a need for an alternate approach to inspecting boiler tubes for corrosion fatigue. The method should be rapid with much better signal-to-noise ratios then the conventional ultrasonic technique.